This study is based on the hypotheses that: 1. Some thyroid carcinomas are epigenetic in nature, and 2. the expression of the embryonal and adult genome of the epigenetic-malignant thyroid cells may be strongly influenced by environmental factors (hormones and nonspecific humoral factors). Certain medullary carcinomas exhibit embryonal genome by secreting ACTH-like substances. Both the embryonal genome, and adult genome controling normal structure and function are sequentially manifested in some human papillary-follicular thyroid carcinomas by forming metastases composed of normal-appearing and functioning thyroid follicles. The same phenomenon was observed in some transplantable papillary-follicular thyroid tumors of the rat (TTTR) when trypsin dispersed cells of these tumors were injected intracardially in the same inbred species of rats. Therefore, the following studies are proposed: 1. Assay of carcino-embryonic antigen (CEA) in tumor extracts and plasma from patients with: a. medullary thyroid carcinoma b. papillary-follicular thyroid carcinomas forming metastases of normal-appearing thyroid follicles, and c. assay of CEA in plasma of the rats and extracts of their transplantable thyroid tumors. 2. Cell culture of cloned TTTR and suitable human thyroid carcinomas to define the structure, function and growth rate of tumor cells displaying embryonal and adult genome. Follicle formation and electromicroscopic structure, metabolism of I131, length of cell cycle and its phases of these cells (H3-thymidine) will be established and evaluated in vitro and in suitable in vivo experiments. It is expected that these studies will: a) determine the value of the CEA assay in diagnosis of certain thyroid carcinomas and b) contribute a pathogenetic (tumorigenic) mechanism to the classification of thyroid carcinoma.